The present invention relates to a critical dimension measurement method and apparatus utilizing an optical microscope and a two-dimensional image sensor such as a CCD camera. In particular, the present invention relates to a very narrow line size measurement method and apparatus suitable for, although not restricted to, measuring a critical dimension, such as a magnetic head track width and a line width of a photomask for semiconductor fabrication, in a contactless manner.
JP-A-59-176605, for example, describes a basic configuration of a size measurement apparatus. A configuration similar thereto is shown in FIG. 11.
In FIG. 11, an image of an object 307 projected by an optical microscope 305 is picked up by a CCD camera 308. A size measurement processor 309 electrically measures sizes of desired sections of the image obtained by the CCD camera 308. The image and values of sizes of the object 307 are displayed on a video monitor 310. Numeral 306 denotes a light source.
FIG. 6 shows the display screen of the video monitor 310 and a luminance distribution of an image of an object (here a magnetic head surface) 307 picked up by the CCD camera 308 of FIG. 11 on an object scanning line 6 (Li) of the video monitor 310. In this case, as is clear from the luminance distribution on the measurement object scanning line 6 (Li), the luminance becomes largest at the pole 5 portion of the magnetic head.
This critical dimension measurement apparatus obtains the size of a line width of an measurement object (in this case, the size of the track width which is the width of the pole 5) such that a video signal corresponding to the scanning line 6 (Li) is stored in a frame memory (not illustrated) provided in the size measurement processor 309 pixel by pixel to determine respective N-divided pixel positions and obtain the line width of the measurement object based on the N-divided pixel positions and their respective luminance values. More specifically, a maximum luminance level 61 and a minimum luminance level 62 in the luminance distribution are assumed to be 100% and 0%, respectively. A positional difference Nab between an a-th pixel and a b-th pixel which each correspond to a luminance level 63 of 50% is obtained. The positional difference Nab is multiplied by a coefficient k determined according to a magnification factor of the optical microscope 305 and a light receiving size of the CCD camera 308 to obtain a value of size M of the object 307 in accordance with equation (1) below. This method is referred to as edge detection method for the sake of convenience.
M=kxc3x97Nabxe2x80x83xe2x80x83(1) 
On the other hand, a minimum size limit in size measurement becomes the resolution of the optical microscope. The resolution xcex1 is represented by equation (2).
xcex1=xcex/(2xc3x97NA)xe2x80x83xe2x80x83(2) 
where xcex is the wavelength, and NA is the numerical aperture of the object lens.
However, the above-described conventional method has a problem that the measurement becomes impossible when the actual size of the object is smaller than the resolution xcex1 of the optical microscope. A microscopic size measurement method for solving this problem is disclosed in JP-B-6-103168 (Japanese Patent No. 1967489).
Hereafter, the problem of the above-described conventional technique will be described by referring to FIG. 7.
FIG. 7 is a diagram showing relations between the resolution xcex1 of an optical microscope and the luminance distribution. In FIG. 7, numeral 34 denotes a luminance distribution obtained by picking up the image of a white line having a width 2xcex1, numeral 37 a minimum luminance level, numeral 38 a maximum luminance level (VH), and numeral 41 a threshold level, that is, an intermediate level (50% level) between the maximum luminance level 38 and the minimum luminance level 37. Furthermore, numeral 35 denotes a luminance distribution of a white line having an width xcex1, numeral 39 a maximum luminance level, and numeral 42 a threshold level (50% level). Furthermore, numeral 36 denotes a luminance distribution of a white line having a width less than xcex1, numeral 40 a maximum luminance level, and numeral 43 a threshold level (50%). As shown in FIG. 7, the luminance distribution 34 of the white line with the width 2xcex1, the luminance distribution 35 of the white line with the width xcex1, and the luminance distribution of the white line 36 with the width less than xcex1 have sizes 53, 54 and 55 according to the edge detection method, respectively. Although the relation 53 greater than 54 holds true, it becomes impossible to determine which one of 54 and 55 is greater or less than the other, which means that, from the luminance distribution of the white line having the width less than xcex1, it is impossible to measure the size of the white line having the line width less than xcex1.
Furthermore, in FIG. 7, numeral 44 denotes a luminance distribution of a black line having a width 2xcex1, numeral 47 a minimum luminance level, and numeral 50 a threshold level (50% level). Furthermore, numeral 45 denotes a luminance distribution of a black line having a width a, numeral 48 a minimum luminance level, and numeral 51 a threshold level (50% level). Furthermore, numeral 46 denotes a luminance distribution of a black line having a width less than xcex1, numeral 49 a minimum luminance level, and numeral 52 a threshold level (50% level). As shown in FIG. 7, the luminance distribution 44 of the black line with the width 2xcex1, the luminance distribution 45 of the black line with the width xcex1, and the luminance distribution 46 of the black line with the width less than a have sizes 56, 57 and 58 according to the edge detection method, respectively. Although the relation 56 greater than 57 holds true, it becomes impossible to determine which is greater, 57 or 58, and the size of the black line having the width less than a cannot be measured.
It is now assumed that the numerical aperture of the object lens having a magnification factor of 100 is NA=0.95.
When a visible-ray optical microscope is used and the light source wavelength is xcexc=0.55 xcexcm, the resolution is, xcex1=0.29 xcexcm;
when an ultraviolet-ray optical microscope is used and the light source wavelength is xcex=0.365 xcexcm, the resolution is, xcex1=0.19 xcexcm; and
when a deep ultraviolet-ray optical microscope is used and the light source wavelength is xcex=0.248 xcexcm, the resolution is, xcex1=0.13 xcexcm. It is impossible to measure objects having the sizes less than these sizes.
In order to address this problem, a method described in JP-B-6-103168 (Japanese Patent No. 1967489) has already been proposed.
FIG. 8 is a diagram for explaining the critical dimension measurement according to this conventional technique. An image of the object 307 shown in FIG. 11 is picked up by the CCD camera 308. A video signal corresponding to one scanning line for size measurement is taken in the size measurement processor 309. Its luminance level is digitized by an A-D converter (not illustrated). The digitized luminance level is stored in a frame memory (not illustrated) formed of a series of storage elements on a pixel by pixel basis. A luminance level characteristic in each pixel position at this time is shown in FIG. 8. It is now assumed that pixel addresses on the frame memory are 0 to N and a luminance level of an address i is Vi. It is also assumed that a maximum value 61 of a stored luminance level 60 is its 100% level and a minimum value 62 of the stored luminance level 60 is its 0% level. A value of a threshold level TL 63 is set to be, for example, a 50% level. Addresses of pixels a and b having the same luminance level as the threshold level TL 63 are obtained. And luminance levels at all addresses between the pixels a and b (i.e., luminances above the threshold level TL) are added up. An integral value S of the luminance level Vi between the pixels a and b is obtained by the following equation (3).                     S        =                              ∑                          i              =              a                        b                    ⁢                      xe2x80x83                    ⁢                      (                          Vi              -                              T                L                                      )                                              (        3        )            
The integral value S has a close proportional relation to the actual size of the object 307. The integral value S is multiplied by a previously calculated coefficient k which is determined by an optical magnification factor of the microscope 305 and so on. A measured size value M of the object 307 is thus obtained by the following equation (4).                     M        =                  k          ⁢                                    ∑                              i                =                a                            b                        ⁢                          xe2x80x83                        ⁢                          (                              Vi                -                                  T                  L                                            )                                                          (        4        )            
The explanation above is directed to a size measurement method in the case where the size of the measurement object is less than the resolution xcex1 of the microscope. In the size measurement method in the case where the size of the measurement object is not less than the resolution xcex1 of the microscope, the measurement method based on the position difference between contours of the object image according to the conventional technique described first can also be used in combination. Accordingly, according to this conventional size measurement method, the size measurement is possible for not only a measurement object having the size not less than the resolution xcex1 but also for a measurement object having the size less than the resolution xcex1. The measurement method according to this conventional technique provides a measured value characteristic 72 shown in FIG. 9. The measured value characteristic 72 is closer to an ideal size value characteristic 71 than a measured value characteristic 70 obtained by using only edge detection components is.
As to this conventional microscopic size measurement, the following fact has been found as a result of actual measurements in the past many years. In the case where the measurement object is a black or light absorptive gap or line width such as a gap length of a magnetic head, it has been already established that there is a good correlation relative to the actual size value (value measured by a scanning electron microscope or SEM for length measurement) as far as up to the width xcex1/3. In the case where the measurement object is a white or light reflective line width such as a track width of a magnetic head, however, the present inventors has discovered that no correlation exists relative to the actual size value (value measured by SEM) at near a width xcex1/2 when only the positional difference between contours and the luminance integral value are used.
An example will now be described by referring to FIG. 10. FIG. 10 is a diagram showing relations between the resolution xcex1 and luminance distribution when an ultraviolet-ray optical microscope is used.
When the wavelength of a light source is xcex=0.365 xcexcm, xcex1 is xcex1=0.19 xcexcm. In the case of a size measurement using a luminance distribution of a white line, a luminance distribution 34 of a white line having the width 2xcex1, a luminance distribution 35 of a white line having the width a, a luminance distribution 36 of a white line having the width 2xcex1/3, and a luminance distribution 80 of a white line having the width xcex1/2 become as shown in FIG. 10. Actual size values and measured values according to the conventional technique in that case are shown in FIG. 10. As seen from this, the measured value obtained from the luminance distribution 36 of the white line having the width 2xcex1/3 becomes 0.13 xcexcm according to the conventional technique, whereas the measure value obtained from the luminance distribution 80 of the white having the width xcex1/2 becomes 0.15 xcexcm. The size relation is thus reversed, and hence measurement is impossible.
An object of the present invention is to provide a critical dimension measurement method and apparatus capable of conducting measurement with respect to even an object having a size of resolution or less of an optical microscope.
In FIG. 10, looking at the maximum luminance in the luminance distribution of a white line for each line width, it can be seen that the maximum luminance 38 (Vmax) in the luminance distribution 34 of a white line having a width sufficiently wider than the resolution xcex1 (here, the width is 2xcex1) is the maximum luminance (VH) serving as a reference in the measurement of white line widths. Further, it can be seen that the maximum luminance 40 in the luminance distribution 36 of the white line having the width 2xcex1/3 is lower than the maximum luminance 39 in the luminance distribution 35 of the white line having the width xcex1 and the maximum luminance 81 in the luminance distribution 80 of the white line having the width xcex1/2 is lower than the maximum luminance 40 in the luminance distribution 36 of the white line having the width 2xcex1/3. In other words, as the white line width gets narrower, the maximum luminance Vmax also becomes lower, from which the present inventors have found that by correcting the measured value M0 (kxc3x97Nab in FIG. 10) obtained by the conventional size measurement method explained with reference to FIG. 11 with the maximum luminance Vmax relative to the reference maximum luminance VH, a measurement value M can be obtained which has a good correlation with the actual size value. More specifically, the present inventors have found that measured values having a good correlation with actual size values can be obtained by multiplying the measured value M0 obtained by the conventional technique by the maximum luminance Vmax and dividing a resultant product by a maximum luminance VH of the luminance distribution 34 obtained by picking up the image of a white line having a width wider than the width xcex1. This is expressed by the following equation (5). As a result, it becomes possible to provide measured values satisfying the relation: white line (xcex1) greater than white line (xcex1/2) greater than white line (xcex1/3) greater than white line (xcex1/4).
M=M0xc3x97(Vmax/VH)xe2x80x83xe2x80x83(5) 
According to the measurement of the present invention, the measured value becomes 0.12 xcexcm for the luminance distribution 36 of the white line having the width 2xcex1/3 and 0.095 xcexcm for the luminance distribution 80 of the white line having the width xcex1/2, from which it is seen that the measured values show a good agreement with the actual values.
A critical dimension measurement apparatus according to one aspect of the present invention, comprising: an optical microscope; an image picking up apparatus coupled to the optical microscope for picking up the image of an object to be measured; a display apparatus for displaying a video signal obtained from the image picking up apparatus; and an operating unit for processing the video signal to calculate a size of a predetermined portion of the object, wherein the operating unit calculates a distance between two points of the predetermined portion of the object substantially coinciding with a predetermined first luminance level, and corrects the calculated distance with information on an extremum of a second luminance level of the predetermined portion.
In one embodiment, the first luminance level is set to be a predetermined level between a maximum value and a minimum value of the luminance level of the predetermined portion of the object.
In one embodiment, the information on the extremum of the second luminance level includes a ratio of a maximum luminance level between the two points corresponding to the first luminance level to a maximum luminance level to be used as a reference in the predetermined portion of said object.
In one embodiment, the size, denoted as M, of the predetermined portion of the object is calculated in accordance with an equation,
M=M0xc3x97(Vmax/VH) 
where M0: a size value detected by an edge detection method, Vmax: the maximum luminance level between the two points corresponding to the first luminance level, VH: a maximum luminance level at a portion greater than a resolution of the optical microscope serving as the reference.
In one embodiment, the object is a track width of a magnetic head.
In one embodiment, the object is a line width of a photomask for fabricating a semiconductor wafer. In one embodiment, the critical dimension measurement apparatus includes a video peak detection circuit for optimizing a quantity of light incident on the image picking up apparatus.
A critical dimension measuring method according to another aspect of the invention comprising: a first step of picking up an image of an object by using an optical microscope and an image sensor, and detecting a luminance of a portion of the object wider than a width equivalent to a resolution of the optical microscope, as a maximum luminance level (VH) to be used as a reference; a second step of detecting a luminance-pixel characteristic of the object on one measurement line which crosses a measurement region of the object, by using the image sensor, and storing the luminance-pixel characteristic in a memory on a pixel by pixel basis; a third step of obtaining a maximum luminance (Vmax) and a minimum luminance (Vmin) from the luminance-pixel characteristic of the object; a fourth step of multiplying a difference between the maximum luminance and the minimum luminance by a predetermined ratio to obtain a threshold level; a fifth step of obtaining a distance (Nab) between pixels corresponding to the threshold level; and a sixth step of calculating a size of the object from a value obtained by multiplying the distance (Nab) by a ratio of the maximum luminance (Vmax) to the reference maximum luminance level (VH).
In one embodiment, the critical dimension measurement method further comprising a step of optimizing a quantity of light incident on the image sensor, before the first step.
In one embodiment, the critical dimension measurement method further comprising a step of repeating the second to sixth steps on a plurality of measurement line, averaging a plurality of sizes of the object obtained at the sixth steps, and thereby calculating a more accurate size of the object.